1. Field of the Invention
The present invention relates to a solid state image pickup device and to a method of manufacturing such a device. More particularly, the present invention relates to a solid state, micro-slim type semiconductor device having an enhanced sensitivity for image pickup, and a method for manufacturing the same.
2. Discussion of the Related Art
A general solid state image pickup device generates an image signal corresponding to a subject. In the device, a photoelectric conversion device (e.g., a photodiode) and a charge coupled device (CCD) are used to convert the image into an electric signal. The CCD transfer signal charge is generated in the photoelectric conversion device in a specific direction, based on changes of electric potential in a substrate.
A general solid state image pickup device includes a plurality of photoelectric conversion regions such as photodiodes (PD); a vertical charge coupled device (VCCD) region, formed between the plurality of photoelectric conversion regions, for transferring charge generated in the photoelectric conversion regions in a vertical direction; a horizontal charge coupled device (HCCD) region for transferring the charge that has been transferred from the VCCD region in a horizontal direction; and a floating diffusion region for sensing and amplifying the charge transferred in the horizontal direction and outputting it to a periphery circuit.
A conventional method for manufacturing a solid state image pickup device will be explained with reference to FIGS. 1a and 1b, which are cross-sectional views showing a structure of a conventional solid state image pickup device illustrating paths of incident light.
As shown in FIG. 1a, first and second p.sup.- -type wells 12 and 13 are formed in an n-type substrate 11. Within wells 12 and 13, an n.sup.+ -type photodiode 14 and an n.sup.+ -type vertical charge coupled device VCCD 17 are formed, respectively. On the n.sup.+ -type photodiode region 14, a p.sup.+ -type surface-isolating layer 15 is formed. A third p.sup.- -type well 16 is formed to surround the n.sup.+ -type VCCD 17. Then, a gate insulating layer 19 is formed on the entire surface of the substrate 11. Next, a transmitting gate 20, an interlayer insulating layer 21, and a metal shading layer 22 are sequentially formed on the gate insulating layer 19 and over the photodiode region 14, such that they are exclusive of the gate insulating layer 19. Subsequently, on the entire surface, there is formed a passivation layer 23, on which a planarizing layer 24 is formed. Thereafter, a microlens 25 is formed on the planarizing layer 24 opposite to the photodiode region 14. A channel stop layer 18 for isolating adjacent pixels is formed to surround the photodiode region 14.
In a conventional solid state image pickup device having the aforementioned structure, light incident through a camera lens is focused toward photodiode region 14 using microlens 25, as shown in FIG. 1a. The light transmitted to the photodiode region 14 is photoelectrically converted into image charge. The image charge is transferred from photodiode region 14 to a horizontal charge coupled device (HCCD) (not shown) in a vertical direction through the VCCD 17, using a VCCD clock signal. The image charge is then transferred through the HCCD in a horizontal direction, and finally is sensed and amplified at a floating diffusion region before being input to a periphery circuit.
In such a conventional solid state image pickup device, light is focused on photodiode region 14 using microlens 25, thus enhancing its sensitivity. However, as shown in FIG. 1b, the microlens 25 is less capable of focusing tilted light toward photodiode region 14. Thus, tilted light generates smear, effectively degrading resultant picture quality. In order to precisely focus incident light on a photodiode region through a microlens, the refractive index and transmissivity should be considered to determine an appropriate thickness of the microlens. It is therefore difficult to selectively change the thickness h. Furthermore, it is difficult to make the conventional device thin while addressing the problems with regard to tilted light.